Perhaps if a previous collar caused an allergic reaction and resulting discomfort, he has learnt to associate them with that. Some dogs cen be really sensitive to such things and don't forget.

As mine are clicker trained, if there was anything that made them anxious, I would desensitize them to it by clicking and treating any tolerant behaviour towards the collar whilst not actually on them yet and would gradually work towards just hanging the undone collar over the top of their neck and then to fastening it loose for a minute or two before taking back off and gradually working up...always being sure to keep within the dogs threshold. The rewards help the dogs anxiety over the collar to diminish and the click is what marks the tolerant behaviour so there is no doubt in the dogs mind what is being rewarded.